1. Field of the Invention
The present invention relates to a technology for bonding a metal terminal to a substrate pad.
2. Description of the Related Art
An automobile has control units mounted thereon for electronically controlling a power train system such as an engine, and a steering system for steering operation. The control unit is demanded to exhibit functionally high reliability. Recently, the automobile has been developed into a highly sophisticated and multifunctional structure, resulting in increased control units installed in the automobile. Meanwhile, the automobile has the limited space for accommodating the aforementioned control units, and accordingly, downsizing (compact structure) of the control unit is highly demanded.
A module for converting and controlling electric power (power module) is installed in the control unit. FIG. 2 illustrates a structure of a generally employed power module substrate. A high-heat generating power device 230 such as a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is mounted on a substrate 210. The power module receives high current application in the range from several tens to hundreds amperes or higher. Because of high heat value, it is important to produce the power module in consideration of conductivity and radiation performance.
The power module is connected to another module through AI wire bonding or Cu terminal solder connection. The aforementioned process, however, has a disadvantage as follows. That is, the upper limit of the wire radius for the Al wire bonding is set to several hundreds μm. Meanwhile, the power module receives application of the electric current in the range from several tens to hundreds amperes or higher.
From the aspect of electric resistance, plural wires have to be connected to a single bonded portion, and accordingly, the resultant structure is no longer compact. Meanwhile, the Cu terminal solder connection allows both values of the width and thickness of the terminal to be increased by the amount in the range from sub mm to several mm to exhibit the high electric conductivity and good radiation performance.
FIGS. 3A and 3B are perspective views illustrating how the module substrates are laminated. FIG. 3A is a perspective view illustrating connection of a terminal for bonding the module substrate to the other. FIG. 3B is a perspective view illustrating a state where the module substrates are bonded using the terminal. FIG. 3B omits elements of the other module substrate except the connection solder portion.
FIGS. 3A and 3B illustrate that Cu terminals 220 are mounted on a substrate 210 with solders 240, and then each one end of the Cu terminals 220 is connected to another module 250 with a solder 260. In this case, the solder connection for bonding the modules is performed by using through hole.
In the aforementioned case, when the same material for forming the solder 240 which is used for connecting the Cu terminal with the substrate is used for forming the solder 260 which is used for connecting the Cu terminal 220 to the other module 250, the problem of remelting the solder 240 for connecting the Cu terminal and the substrate will occur.
For solving the aforementioned problem, the material with higher fusing point than that of the material for forming the solder 260 for connecting the modules is used for forming the solder 240 for connecting the Cu terminal with the substrate. In this case, however, the mount structure, material, and the connection process will be restricted. Recently, ultrasonic bonding of the Cu terminal has been drawing attention as the solution.
Japanese Published Unexamined Patent Application Nos. 2007-173363, 2007-088030, and 2002-280416 disclose the connection technology through ultrasonic bonding in place of the soldering.
FIGS. 4A and 4B schematically illustrate generally employed ultrasonic bonding. FIG. 4A is a sectional view, and FIG. 4B is a plan view corresponding to FIG. 4A with an ultrasonic tool removed. A Cu terminal 320 is disposed on a substrate pad 313. The ultrasonic oscillation is applied to the Cu terminal in a direction parallel with the substrate surface while being pressed from above.
Renewed surfaces of the interface between the terminal and the pad formed by removing adhered matter and oxide are bonded. In the aforementioned ultrasonic bonding, the solder which has been applied before is not remelted. The Cu terminal may be directly bonded to the substrate pad 313, thus providing the compact bonded structure with high electric conductivity and high radiation performance.
In the ultrasonic bonding of the terminal to the substrate pad as described above, abrasion 114 or a crack 115 may occur in the pad just below the terminal edge during the bonding as illustrated in FIG. 5, resulting in deteriorated reliability. The terminal edge is designated with a bold dashed line 321 shown in FIG. 4B. As high frictional force is exerted to the pad 113 below the edge of a terminal 120, a shear force is caused under the friction. The abrasion 114 and the crack 115 occur on the line of the dotted line 321, which is perpendicular to the moving direction of the ultrasonic wave as shown in FIG. 4B.
The shear force is proportional to the frictional force, and the frictional force is proportional to the pressure, which may cause the abrasion and crack just below the terminal edge where the pressure is locally increased. The crack may be avoided by lowering ultrasonic pressurization. However, this will reduce the bonded area, and accordingly further reduce the bonding strength, conductivity and radiation performance.
The region which does not cover the terminal edge may be ultrasonic bonded as shown in FIGS. 7A and 7B. FIG. 7A is a sectional view, and FIG. 7B is a plan view corresponding to FIG. 7A with the ultrasonic bonding tool removed. In the structure as shown in FIGS. 7A and 7B, the pressure applied to the portion of the pad 113 just below the edge of the terminal 120 is reduced for avoiding the abrasion and crack in the pad.
The region which does not cover the terminal edge is ultrasonic bonded by positioning the terminal edge outside a region 131 to be pressurized by an ultrasonic bonding tool 130. The structure shown in FIGS. 7A and 7B may be formed by enlarging the terminal (increasing thickness and length), or reducing the bonded area.
Enlargement of the terminal 120 may prevent pitch reduction, which fails to make the module compact. Meanwhile, the area reduction may deteriorate the conductivity and radiation performance. The generally employed technique for the ultrasonic bonding of the terminal to the substrate pad has difficulties in satisfying those requirements at a time, that is, high reliability, bonding strength, conductivity, and radiation performance without causing a crack for achieving the compact structure resulting from the pitch reduction.